1. Field of the Invention
The present invention relates to apparatus and methods for combining audio compression and feedback cancellation in audio systems such as hearing aids.
2. Description of the Prior Art
Mechanical and acoustic feedback limits the maximum gain that can be achieved in most hearing aids. System instability caused by feedback is sometimes audible as a continuous high-frequency tone or whistle emanating from the hearing aid. Mechanical vibrations from the receiver in a high-power hearing aid can be reduced by combining the outputs of two receivers mounted back-to-back so as to cancel the net mechanical moment; as much as 10 dB additional gain can be achieved before the onset of oscillation when this is done. But in most instruments, venting the BTE earmold or ITE shell establishes an acoustic feedback path that limits the maximum possible gain to less than 40 dB for a small vent and even less for large vents. The acoustic feedback path includes the effects of the hearing aid amplifier, receiver, and microphone as well as the vent acoustics.
The traditional procedure for increasing the stability of a hearing aid is to reduce the gain at high frequencies. Controlling feedback by modifying the system frequency response, however, means that the desired high-frequency response of the instrument must be sacrificed in order to maintain stability. Phase shifters and notch filters have also been tried, but have not proven to be very effective.
A more effective technique is feedback cancellation, in which the feedback signal is estimated and subtracted from the microphone signal. One particularly effective feedback cancellation scheme is disclosed in patent application Ser. No. 08/972,265, now U.S. Pat. No. 6,072,884 entitled xe2x80x9cFeedback Cancellation Apparatus and Methods,xe2x80x9d incorporated herein by reference.
Another technique often used in hearings aids is audio compression of the input signal. Both single band and multiband dynamic range compression is well known in the art of audio processing. Roughly speaking, the purpose of dynamic range compression is to make soft sounds louder without making loud sounds louder (or equivalently, to make loud sounds softer without making soft sounds softer). Therefore, one well known use of dynamic range compression is in hearing aids, where it is desirable to boost low level sounds without making loud sounds even louder.
The purpose of multiband dynamic range compression is to allow compression to be controlled separately in different frequency bands. Thus, high frequency sounds, such as speech consonants, can be made louder while loud environmental noisesxe2x80x94rumbles, traffic noise, cocktail party babblexe2x80x94can be attenuated.
Patent application Ser. No. 08/540,534, entitled xe2x80x9cDigital Signal Processing Hearing Aid,xe2x80x9d incorporated herein by reference, gives an extended summary of multiband dynamic range compression techniques with many references to the prior art.
Patent application Ser. No. 08/870,426, entitled xe2x80x9cContinuous Frequency Dynamic Range Audio Compressor,xe2x80x9d incorporated herein by reference, teaches another effective multiband compression scheme.
A need remains in the art for apparatus and methods to combine audio compression and feedback cancellation in audio systems such as hearing aids.
The primary objective of the combined audio compression and feedback cancellation processing of the present invention is to eliminate xe2x80x9cwhistlingxe2x80x9d due to feedback in an unstable hearing aid amplification system, while make soft sounds louder without making loud sounds louder, in a selectable manner according to frequency.
The feedback cancellation element of the present invention uses one or more filters to model the feedback path of the system and thereby subtract the expected feedback from the audio signal before hearing aid processing occurs. The hearing aid processing includes audio compression, for example multiband compression.
As features of the present invention, the operation of the audio compression element may be responsive to information gleaned from the feedback cancellation element, the feedback cancellation may be responsive to information gleaned from the compression element, or both.
A hearing aid according to a first embodiment of the present invention comprises a microphone for converting sound into an audio signal, feedback cancellation means including means for estimating a physical feedback signal of the hearing aid, and means for modelling a signal processing feedback signal to compensate for the estimated physical feedback signal, subtracting means, connected to the output of the microphone and the output of the feedback cancellation means, for subtracting the signal processing feedback signal from the audio signal to form a compensated audio signal, a hearing aid processor including audio compression means, connected to the output of the subtracting means, for processing the compensated audio signal, and a speaker, connected to the output of the hearing aid processor, for converting the processed compensated audio signal into a sound signal.
In a second embodiment, the feedback cancellation means provides information to the compression means , and the compression means adjusts its operation in accordance with this information. For example, an increase in the magnitude of the zero coefficient vector can indicate the presence of an incoming sinusoid, which is likely due to feedback oscillations in the hearing aid. The maximum gain of the audio compression at low levels can be reduced if the feedback cancellation means detects an increase in the magnitude of the zero coefficient vector.
In a third embodiment, the compression means provides information, for example input signal power levels at various frequencies, to the feedback cancellation means, and the feedback cancellation element adjusts its operation in accordance with this information. For example, the feedback cancellation adaptation constant can be adjusted based upon the power level of one or more of the frequency bands of the audio compressor. For example, the adaptation time constant of the feedback cancellation element could be adjusted based on the output of one of the compression bands or a weighted combination of two or more bands.
In a fourth embodiment, the compression means provides information to the feedback cancellation means, and the feedback cancellation means provides information to the compression means, and each element adjusts its operation in accordance with the information obtained from the other.